Functional integral construction of the BRST invariant string ground state

The Euclidean functional integral prescription is used to construct the ground-state wavefunction of the closed bosonic string including ghost contributions. The authors work in 26 dimensions where the sole contribution to the wavefunction comes from the string and ghost classical actions as well as the tachyonic centre of mass motion. It is shown that the ghost action must include a surface term to be consistent with the ghost boundary conditions. The result obtained represents the usual string plus ghost ground state and is BRST invariant. (author)